apostate

(ἀποστάτης, a rebel, renegade), a term used, in its strict sense, by ecclesiastical writers, to designate one who has, either wholly or in part, left the true faith to embrace a false belief, or who has forsaken any holy profession to which he was bound by solemn vows. The term apostate is, in Church history, applied by way of emphasis to the Emperor Julian, who, though he had been nominally Christian when he came to the throne, renounced the Christian religion, and used every means in his power to reestablish paganism in the empire. SEE HERETIC.

One who is guilty of desertion of one’s faith, religion, party, or principles